This invention relates generally to methods and apparatus for computed tomography (CT) imaging of objects, and more particularly to methods and apparatus for producing compensated tilted, helically scanned CT images of objects.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In known multislice CT imaging systems, gantry tilt is disabled for helical scans. When the gantry is tilted, degraded image quality results because the isocenter is inherently detector row dependent. The isocenter is shifted relative to the patient scanning axis, and the amount of shift depends upon detector row, amount of gantry tilt, and projection angle.
For example, and referring to FIG. 3, an axial scan of a human skull is depicted for a 1.25 mm slice thickness. The gantry was tilted at -20.degree. and no projection weighting was applied in the reconstruction of the image of FIG. 3. This image was reconstructed with a 15 cm field of view (FOV) and a high-resolution algorithm to illustrate fine structural details of the scanned phantom. This image serves as a standard for image quality evaluation, as no degradation is present in a tilted axial scan mode.
For comparison, the same phantom used to generate the image of FIG. 3 was scanned in a helical mode with 1.25 mm slice thickness at 3:1 helical pitch to generate the image shown in FIG. 4. For generating this image, a helical scan was performed in which the patient table was moved 3.75 mm per gantry rotation. The image of FIG. 4 was reconstructed without z-smoothing, and the same reconstruction parameters were used as those of FIG. 3. The fine bony structures are blurred in FIG. 4 as a result of the isocenter shift.
It might be expected that isocenter shift could be corrected by a compensation algorithm that interpolates projection data to a desired location. Such algorithms turn out to be somewhat effective in combating double blurring structures. However, spatial resolution of reconstructed images is compromised because the interpolation kernel acts as a low pass filter to the projection data. To partially compensate for resolution degradation, a high frequency kernel boost can be introduced to "sharpen" the image. However, spatial resolution cannot be recovered. An example of a "sharpened" image is shown in FIG. 5, which depicts a "sharpened" version of the same scan as shown in FIG. 4. Compared to the axially-scanned image of FIG. 3, the loss of spatial resolution is obvious. (The rotation of FIG. 5 is due to implementation artifacts, and is unrelated to the reconstruction and "sharpening" algorithms.)
It would therefore be desirable to provide methods and apparatus for filtering data to produce compensated tilted, helically scanned CT images of objects.